The proposed research involves measuring the effect of radiation on cells and determining the mechanisms by which cells deal with damage to their genetic material. Specifically, cultures of the radioresistant organism Tetrahymena pyriformis will be irradiated with X-rays, and the DNA molecular weight distribution will be monitored during the repair process by viscoelastometry and field inversion gel electrophoresis. Cell survival and morphology will be assayed microscopically, and DNA repair enzymology will be investigated through the use of various enzyme inhibitors. Preliminary results indicate that Tetrahymena pyriformis has a very efficient mechanisms for repairing both single and double strand breaks in DNA which result from large doses of X-rays, that this repair system is inducible, that various components of the repair system can be inhibited by exogenously-added chemicals, and that the enzyme ADP-ribosyl transferases plays an important role in the repair process. Furthermore, the DNA damage and repair process is observable both at the macromolecular level (via DNA molecular weight analysis using viscoelastometry) and at the cellular level (via microscopic examination of cellular morphology). We intend to pursue these initial observations by determining the optimal repair conditions (dosage and timing), conducting the necessary controls (effects of inhibitors in the absence of irradiation(, examining the effect of other inhibitors or inducers, studying entire DNA molecular weight distributions and intermediates by field inversion gel electrophoresis, and monitoring the largest molecules by viscoelastometry. Not only will these studies provide additional information regarding how cells deal with radiation damage to their DNA, but if the inducibility of a enzymatic repair system in Tetrahymena can be confirmed and more fully characterized, the hypothesis of radiation hormesis will be considerably strengthened. This hypothesis states, in effect, that small doses of radiation are actually beneficial to the health of an organism. In this case small initial doses of X-rays may induce a DNA repair system in Tetrahymena cells that allows them to deal with subsequent large doses of X-rays in a more productive, efficient, healthy way.